Despite the fact that aluminium is a reactive metal, it is a stable in air as well as in water. This is due to the formation of thin layer of oxide on the surface of aluminium metal which makes it passive for further attack. The layer is so useful, that in industry, it is purposely deposited by an electrolytic process called anodising. Reaction of aluminium with oxygen is highly exothermic and is called thermite reaction.
`2Al_((s)) +(3)/(2) O_(2(g)) rarr Al_(2)O_(3(s)) Delta_(f) H^(Ө) = -1670 kJmol^-1`
This reaction finds application for welding of metal and in the metallurgical extraction of many metals from their oxides.
The limitation is that to start the reaction, high temperature is required for which an ignition mixture is required.
Aluminium becomes passive in :

Options

A. `H_(2)CrO_(4)`
B. `conc HNO_(3)`
C. `HClO_(4)`
D. `NaOH`

Answer

A, B, C

Explanation

`H_(2) CrO_(4)`, conc `HNO_(3)` and `HClO_(4)` behave as strong oxidising agents and hence oxidise `Al` to `Al_(2)O_(3)`. During this a layer of `Al_(2)O_(3)` gets deposited on the surface of `Al`, which makes `Al` passive in nature.